Structural members with large unidirectional rigidities are known (see DE 195 29 476 A1, EP 0 758 607 A2). For example, EP 0 758 607 A2 discloses a wing with shear resistant wing shells made of fibre composites for aircraft. In the wing with shear resistant wing shells made of fibre composites, particularly fibre-reinforced plastics, members taking up tensile and compressive forces are mounted inside the wing shells and have unidirectional fibres extending longitudinally of the wing. Spaced stringers are formed inside the wing shells, longitudinally of the wing, their fibre component being formed by a fibre skin joined to the fibre skin of the wing shell. Unidirectional fibre bundles, embedded with thrust, resistance in the synthetic resin matrix of the wing shells, are arranged between the spaced stringers. They extend longitudinally of the wing and have a substantially rectangular cross-section. The space between two stringers to receive a plurality of fibre bundles is divided widthwise by intermediate walls extending parallel with the stringers. The fibre component of the stringers and/or intermediate walls is formed by folding at least the inner fibre layer of the torsion skin of the flight shell. The dividing walls may be provided with skin sections lying on top of the fibre bundles.
Fibre-reinforced structures are normally made up of multi-directional laminates. The dimensioning and also the fibre orientation in the individual laminate layers follow elongation criteria, such that a predetermined elongation value must not be exceeded anywhere within the structure. The limit for elongation of fibre-reinforced structures which has found acceptance for aircraft structural members, according to the application, is an elongation of up to .epsilon.=0.4% with the maximum limit load and up to .epsilon.=0.6% with the ultimate load.
This limit to elongation in the structural members is based on the assumption that the skin covering the load-bearing fibres, generally a plastic matrix, will not crack or suffer other damage from operational causes, which might lead to failure of the structural member, up to those values. So here the function of the plastic matrix is not only to keep the structural member in shape and support the individual fibres, which would not be able to bear loads without that support, particularly under compressive stress. The matrix is also the only load-transferring member in the structure for stresses acting across the laminate planes.
Unidirectional cross-sections of large area, e.g. fibre bundles, undergo large transverse elongation when loaded, causing transverse tensile and transverse compressive stresses in the shell e.g. of a plane load-bearing structure. Such stresses occur wherever transverse elongations of any kind are impeded. In a component with large unidirectional rigidities this leads to constructional limitations and over-dimensioning of rigidities which are not oriented in the direction of the main load.
There is therefore needed in the art structural members of the generic type with large unidirectional rigidities, in which transverse tensile stresses are reduced by the provision of constructional measures.